It has been shown that certain conjugated polymers show a relatively high quantum efficiency for the radiative decay of singlet excitons. Of these, poly-p-phenylene vinylene (PPV) can be prepared via a solution-processible precursor polymer, and although itself intractable and not easily processed, can be prepared in the form of thin films of high quality by thermal conversion of the as-prepared films of the precursor polymer. Details of this general synthesis method are given in "Precursor route poly(p-phenylene vinylene): polymer characterisation and control of electronic properties" D. D. C. Bradley, J. Phys. D: Applied Phys. 20, 1389 (1987), and "Spectroscopic and cyclic voltammetric studies of poly(p-phenylene vinylene) prepared from two different sulphonium salt precursor polymers", J. D. Stenger-Smith, R. W. Lenz and G. Wegner, Polymer 30, 1048 (1989). Measurements of photoluminescence, PL, have been reported by for example "Optical Investigations of Conjugated Polymers" R. H Friend, J. Molecular Electronics, 4, 37 (1988), and "Photoexcitation in Conjugated Polymers" R. H. Friend, D. D. C. Bradley and P. D. Townsend, J. Phys. D 20, 1367 (1987). In our earlier International Patent Application No. PCT/GB90/00584 (Publication No. PCT/WO90/13148) films of PPV are disclosed as being useful as the emissive layer in a structure exhibiting electroluminescence (EL). This structure requires injection of electrons and holes from either side of the active (i.e. emissive) region of the film, and various metallic contact layers can be used. In sandwich-like structures, and for emission from the plane of the device, one of these should be semi-transparent.
The advantages of using polymers of this type as the emissive layer in EL structures include:
(a) ease of fabrication of large area structures. Various methods are available for solution-processing of the precursor polymer, including spin-coating from solution which is the preferred method, and dip-coating; PA1 (b) intractability of the polymer film, giving desirable strength, resistance to degradation from heat and exposure to oxygen, resistance to structural changes such as recrystallisation and shrinkage, and resistance to ion migration; PA1 (c) intrinsically good properties for luminescence, including low densities of charges and/or spin-carrying defects. PA1 (a) reacting a quantity of a first monomer with a quantity of a second monomer in a solvent comprising a mixture of water and an alcohol; PA1 (b) separating the reaction product therefrom; PA1 (c) dissolving the reaction product in an alcohol the same as or different from said first mentioned alcohol; PA1 (d) forming from the result of step (c) a conjugated polymer film the quantities in step (a) being selected so that in the conjugated polymer the semiconductor bandgap is controlled so as to control the optical properties of the copolymer.
However, there is some evidence that the quantum yield for radiative decay of the excited states is lowered through their migration to non-radiative decay centres, see for example "Radiative and Non-Radiative Recombination Processes in Photoexcited Poly(p-phenylenevinylene)", D. D. C. Bradley, R. H. Friend, K. S. Wong, W. Hayes, H. Lindenberger and S. Roth, Springer Solid State Sciences, 76, 107 (1987), and "Light-Induced Luminescence Quenching in Precursor-Route Poly(p-phenylenevinylene)" D. D. C. Bradley and R. H. Friend, J. Phys. CM 1, 3671 (1989).